Driveway Access Safety Assessment.pdfWilliam Popp Associates
May 8, 2014
To: Mr. Bertrand Hauss, P.E.
City of Edmonds Transportation Engineer
C/o: Mr. Dan Del Duca
Clear Vision Homes, Inc.
Renton, WA
From: William Popp Jr.
Senior Transportation Engineer
William Popp Associates
Subject: Edmonds Short Plat: 18227 801h Ave W
Three Single -Family Residential Lots
City of Edmonds, WA
Transportation Engineers/Planners
(4-25) 401-1030
FAX (425) 401-2125
e-mail: info@wmpoppassoc.com
S
1,3
The following report was prepared to identify the driveway access operations and safety
for a proposed short plat located in the northeast vicinity of the City of Edmonds. The
plat is located at 18227 80th Ave W, TPN 00370800101400. The proposed layout
includes dividing one lot into three, with the existing house to remain. The two new lots
would be on the east side of the parcel east of the existing home. This layout is shown in
Attachment 1.
The property does not have enough lot area to share a driveway for all three lots and still
meet the 12,000 square foot minimum lot size requirement for each proposed lot. The
city prefers one access point for safety reasons, however the city would entertain allowing
two access points if a traffic engineering study would support it.
The proposed short plat design includes driveway stems on both the north and south sides
of the parcel, see Attachment 1. The new easterly most lot, Lot 3, would gain access to
801h Ave W via a joint easement driveway (shared with the existing home, Lot 1) on the
14-400 Building e Suite 206 e 14400 Be] -Red Road o Bellevue, WA 98007
Driveway Access Assessment Study (5/8/14) 18227 801x' Ave W
north end of the property, and the new middle lot (Lot 2 between existing Lot 1 and the
easterly most Lot 3) would gain access to 801h Ave W via a driveway on the south end of
the parcel.
Roadway Inventory
801h Ave W in the vicinity of the site is a two-lane two-way street with 10 to 11' lanes,
and variable width paved shoulders (nominal widths). Traffic control consists of side -
street stop signs on abutting local streets. There is an all -way stop at 186th St SW (four-
way) and a stop sign at Olympic View Drive where 801h Ave W turns into 180th St SW.
A survey of 80th Ave W was conducted by Lanktree Land Surveying, Inc. in April 2014.
A detailed review of the survey and sight distance is discussed later.
The posted speed limit on 801h Ave W is 25 mph. The pavement is in excellent condition
(per WADT_PSC; 96 to 100 score). It is important to note that there is a steep grade
(+15% northbound) between 184th St SW and just south of the subject property,
approximately 250 feet in length. There is a vertical curve near the site, and north of the
site the roadway grade is -5% northbound. There are no horizontal curves on 80th Ave W,
except at the north end where the roadway turns east and becomes 180th St SW.
According to the City's 2009 Comprehensive Plan, this roadway is currently identified as
a "local street" (Figure 3-2; Existing Federal Functional Classifications) but is
recommended as a "Collector" between 200th and Olympic View Dr (Figure 3-3;
Recommended Road Functional Classifications). The City's definition of a Collector
includes, among other criteria, traffic volumes that range between 1,000 and 5,000 ADT.
The most recent count on 80th Ave W is 2,100 ADT, thus the route would pass that
measure of a "Collector".
801h Ave W is not a designated bikeway nor is it in the long range plan. In addition there
are no sidewalk amenities and there are no current designated paved walkways along it.
However, it should be noted that a long range planned sidewalk improvement (5'
sidewalk with curb and gutter west side only, 3000 feet) is identified in the Comp Plan
for this roadway (Pedestrian project L6).
There are no traffic calming devices on 80th Ave W in the project vicinity, and none are
planned. It should be noted also that per City guidelines traffic calming features in
general are only permitted on local streets.
Selected photos both northbound and southbound along 80th Ave W are shown in
Attachment 2.
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Driveway Access Assessment Study (5/8/14) 18227 80`x' Ave W
Nearby Residential Driveways
The adjacent parcel to the north and to the south (east side of 801h Ave W) both are
occupied with single-family homes and their direct driveway accesses to 801h Ave W are
approximately 1' and 14' from their nearby driveway edge to their respective abutting
property lines. The abutting south lot also has a gravel parking area that is about P from
the property line.
There are three lots occupied with single-family homes on the west side of 801h Ave W
and each has single driveway access.
Because of the closeness of the north abutting driveway to the property line, the new
shared Lot 1 and 3 project street access driveway is located as far as possible from that
line, such that it shares a portion of the existing residential access to the street.
Site Trip Generation
There are only three lots as part of this short plat, one includes an existing home. Thus
simply stated, there will be 3 trips maximum during peak hour periods. Two of these
would likely be at the north driveway and one at the south driveway. For daily traffic
volume projections, there would be a maximum of 20 ADT at the north driveway and 10
ADT at the south driveway.
Sight Distance
There are two sight distance parameters generally applicable to site development. One is
intersection (or entering) sight distance (ISD/ESD) from project driveways and the other
is stopping sight distance (SSD).
Intersection sight distance requirements are the design parameters set forth in order to
provide sufficient sight distance for entering vehicles such that they do not impede the
mainline traffic speed and in -turn do not reduce the capacity of the roadway. In general, it
is understood that intersection sight distance relates more to driver comfort and roadway
capacity rather than safety. It should be noted that Snohomish County EDDS notes that
ISD shall not apply for non -arterial intersections where the minor road or access is 80
ADT or less.
In this specific case, 80`h Ave W is a non -arterial and the driveway ADT is less than 80.
Therefore, for low volume driveways accessing low volume streets, available sight
distance at the project site access points is typically based solely on stopping sight
distance requirements per AASHTO. In general, the available stopping sight distance on
a roadway should be sufficiently long to enable a vehicle traveling at or near the design
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Driveway Access Assessment Study (5/8/14) 18227 801x' Ave W
speed to stop before reaching a stationary object in its path. In this particular case the
stopping sight distance is measured based on the sight line between the entering vehicle's
motorist eye height from the driveway setback 14.5 feet from edge of traveled way and
the approaching vehicle's motorists eye height; both eye heights are assumed to be 3.5
feet above ground.
The minimum acceptable values for safe stopping sight distance were obtained from the
AASHTO A Policy on Geometric Design of Highways and Streets 2004, Exhibit 3-1 and
adjusted for grade based on AASHTO Equation 3-3: d=V2/[30((a/32.2) ± G)]
In general the minimum SSD is 155 feet at 25 mph for a roadway grade between -3% and
+3%.
The sight distance conditions for left and right exiting viewing scenarios at each driveway
were surveyed given the AASHTO parameters of a driveway vehicle eye height of 3.5
feet above grade and 14.5 feet set back from edge of traveled way (fog line in this case) to
the center of the lane for the approaching vehicle, with an eye height of 3.5 feet. The
sight lines that were reviewed are shown in plan view in Attachment 3. The roadway
profile of 801h Ave W through the project vicinity is shown in Attachment 4. In
Attachment 4, point A is on 80th Ave W at the north driveway location, and point B is at
the south driveway location.
Each of the four sight lines reviewed are discussed and summarized below:
North Driveway (see Attachment 5), Location of driveway for shared access with
Lot 1 and 3 is assumed to be 30 feet south from north property line; 10' wide
driveway.
a. Looking North (sight line A'C).
A' is the location on the north driveway 14.5 feet back from fog line of 80th Ave W.
The point C is the middle of the southbound lane 250 feet from the north driveway,
The sight line looking north from the north driveway was measured to be more than
adequate. The sight line was measured up to 250 feet without obstruction, and likely
further but not measured. Based on the grade of +6% southbound and a posted speed
of 25 mph the minimum SSD is 143 feet. At 35 mph with same grade, the minimum
SSD is 229 feet.
b. Looking South (sight line A'D)
Again, A' is the location on the north driveway 14.5 feet back from fog line of 80th
Ave W. The point D is the middle of the northbound lane where an approaching
vehicle is first visible. The sight line looking south from the north driveway was
measured to be 197 feet. The grade of 801h Ave W approaching the vertical curve is
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Driveway Access Assessment Study (5/8/14) 18227 80"Ave W
+15%. However, the grade from when the approach vehicle is first seen by the
driveway motorist is -0.8% or essentially 0% since it falls within the ±3% condition.
Based on this, the minimum SSD for 25 mph is 155 feet, thus it is concluded the
available SSD is adequate, The available SSD accommodates a maximum speed
condition of 32 mph.
2. South Driveway (see Attachment 6). Location of driveway for Lot 2 is assumed to
be at north end with center of 10' wide driveway 20 feet north of south property line.
a. Looking North (sight line B'E)
B' is the location on the south driveway 14.5 feet back from fog line of 801h Ave W.
The point E is the middle of the southbound lane 250 feet from the south driveway.
The sight line conditions looking north from the south driveway are similar to the
north driveway in that the measured sight distance was found to be more than
adequate. The sight line was measured up to 250 feet without obstruction, and likely
further but not measured. To reiterate the north driveway scenario, based on the
grade of +6% southbound and a posted speed of 25 mph the minimum SSD is 143
feet. At 35 mph with same grade, the minimum SSD is 229 feet.
b. Looking South (sight line B'F)
Again, B' is the location on the south driveway 14.5 feet back from fog line of 80th
Ave W. The point F is the middle of the northbound lane where an approaching
vehicle is first visible. The sight line looking south from the south driveway was
measured to be 143 feet. The grade of 80th Ave W approaching the vertical curve is
+15%. The approaching vehicle can be first seen near the top of the 15% grade.
Based on a 15% grade northbound and a posted speed of 25 mph, the minimum SSD
is 134 feet. However, the grade from when the approaching vehicle is first visible by
the driveway motorist is approximately 4% overall but varies significantly from 15%
at the visual onset to -3% at the driveway. Based on an average grade of 4% through
the vertical curve, the minimum SSD for 25 mph is 146 feet, thus available sight
distance is short 3 feet of minimum. With the available 143 feet of sight distance
and a grade of 4%, the resultant maximum speed condition is 24.6 mph.
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Driveway Access Assessment Study (5/8/14) 18227 80`x' Ave W
Accident History
According to the City's 2009 Comprehensive Plan, 801h Ave W is not identified as a high
accident corridor.
Findings and Conclusions
Based on the foregoing analysis for the proposed 18227 801h Ave W short plat, the
following traffic findings and conclusions are presented;
• The average weekday volume on 80th Ave W is approximately 2,100 vpd which
typically suggests a PM peak hour volume of approximately 210 vph. Since the
driveway volume would only be 1 to 2 vehicles during peak hour conditions, the
intersection level of service for the driveways is clearly LOS A. A formal level of
service analysis was not conducted given the low volumes.
• The available sight distance at the proposed north driveway for vehicles exiting the
parcel (left and right turn out) is calculated to exceed the safe stopping sight distance
requirements of 155 feet per 2004 AASHTO for 25 mph posted speed limit. The
measured sight line exceeds 250 feet looking north and is 197 feet looking south.
The available sight distance at the proposed south driveway for a vehicle exiting the
parcel is calculated to exceed the safe stopping sight distance requirements per 2004
AASHTO for 25 mph posted speed limit looking north; the measured sight line
exceeds 250 feet.
Looking south from the proposed south driveway, the available sight distance is
restricted due to the vertical curve on 80th Ave W, and is measured to be 143 feet.
The uphill grade on 80th Ave W is approximately 15% and the northbound
approaching vehicle can first be seen from the driveway at the top of the grade just
before the vertical curve. The minimum required stopping sight distance for 25 mph
with a +15% grade is 134 feet. However, the grade where the approaching vehicle is
first visible to the driveway vehicle varies through the vertical curve and the average
grade is approximately +4%. For this, the minimum required stopping sight distance
for 25 mph is 146 feet, thus the available SSD is 3 feet short in this instance.
The sight lines as surveyed from both driveway setbacks were free of sight line
obstructions, including parked vehicles and vegetation.
The existing 80th Ave W length of vertical curve as measured from centerline profile
is approximately 135 feet, and the algebraic difference in intersecting grades is 12
(A=15%-3%). According to AASHTO design criteria for vertical curves, the rate of
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Driveway Access Assessment Study (5/8/14) 18227 80'x' Ave W
vertical curvature is 11.2 (K) which per AASHTO Exhibit 3-72 corresponds to a
design speed of 25 mph and a stopping sight distance of 155 feet. The SSD as
measured from the profile is approximately 120 feet for a 3.5 eye height and 29 object
height. This would suggest the actual design speed applicable for measured SSD is
between 20 and 25 mph and likely closer to 20 mph.
• According to the 2009 Comprehensive Plan, 801h Ave W is not identified as a high
accident corridor.
• The proposed two driveways are very low volume and thus conflicts between entering
and exiting movements and the north and south adjacent and opposite side of street
driveways would be extremely infrequent.
There are existing warning signs stating "Limited Sight Distance" W14-4 (24x24)
approximately 250 feet in advance of the vertical curve in both directions. This sign
is no longer recognized in the 2009 MUTCD.
Recommendations
The mitigation recommendations for the 18227 80th Ave W three -lot short plat are as
follows:
1. The shared access for Lot 1 and Lot 3 should be located at least 10 feet south from the
neighboring driveway to the north. In the current plat design, this driveway is located
approximately 29 feet from north property line to the center of driveway.
2. The new Lot 2 should have sole access along the south side of the parcel. The
driveway throat adjacent to the street should be located as far north within the new
Lot 2 access to 801h Avenue W as possible. It is recommended the center of driveway
be 20 feet from the south property line.
The existing warning signs stating "Limited Sight Distance" W14-4 (24x24) located
approximately 250 feet in advance of the vertical curve in both directions should be
removed. Alternatively, they could remain as a grand fathered condition to be used as
advance warning of limited sight distance for the approaching the vertical curve, to be
augmented with Recommendation #4.
4. Applicable to this specific vertical curve situation, it is recommended that a new sign
the "Hill Blocks View" W7-6 (24x24) be placed in advance of the vertical curve in
both directions. In addition, it is recommended that an Advisory Speed W13 -1P
(18x18) sign indicating a speed of 20 mph be installed as a supplemental sign below
the new W7-6. The location of these signs shall be per MUTCD Table 2C-4 -- given
a posted speed of 25 mph and an advisory speed of 20 mph, the MUTCD stats that
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Driveway Access Assessment Study (5/8/14) 18227 801x' Ave W
"No suggested distances are provided for these speeds, as the placement location is
dependent on site conditions and other signing. An alignment warning sign may be
placed anywhere from the point of curvature up to 100 feet in advance of the curve.
However, the alignment warning sign should be installed in advance of the curve and
at least 100 feet from any other signs." Any other signs are presumed to only be
warning or regulatory type signs on 801h Ave W.
ATTACHMENTS:
L Short Plat Parcel Map
2. Roadway pictures
3. Site and 80`h Ave W Plan View Surveyed Sight Lines for Profiles
4. 801h Ave W Roadway Centerline Profile through project vicinity
5. Sight Distance Profile North Driveway Location
6. Sight Distance Profile South Driveway Location
William Popp Associates Page 8
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